In pressurized water reactors for the nuclear production of power, a pressurized fluid is passed through the reactor core and, after being heated in the core, is passed through heat transfer tubes that are positioned in a secondary side of a steam generator. In the secondary side of the steam generator, the heat transfer tubes transfer heat to a secondary fluid to produce steam that is then used to operate a turbine for production of electrical power.
During the operation of the steam generator, impurities find their way into the secondary fluid and tend to concentrate in flow restricted regions in the secondary side of the steam generator. These restricted regions may result from the accumulation of deposits within the generator. The concentrated solutions in the restricted regions can lead to accelerated corrosion of the heat transfer tubes and structural components.
In an effort to prevent the accumulation of deposits in the secondary side of the steam generator, many approaches have been used. One approach has been to blow down the steam generator to remove as much of the impurities as possible from the secondary fluid and dispose of the same. Even with the use of such an approach, however, deposits are still found to be accumulated in the secondary side of the steam generator.
Since deposits are still accumulated in the secondary side of the steam generator, flushing operations have been proposed to periodically remove as much of the dissolved impurities from the flow restricted areas as possible. Such a flushing operation may be effected by introducing a quantity of water into the secondary side of the steam generator while the pressureized water reactor system is at cold shutdown, applying a nitrogen over-pressure, heating the steam generator to about 140.degree. C. using the reactor coolant pumps, and then depressurizing the generator by opening of power-operated relief valves. The valves are subsequently closed and the cycle is repeated. Such a procedure helps to remove sludge from the tubesheet and from crevices found in the secondary side.
Even with the use of such a flushing operation, however, the removal of concentrated solutions of impurities from flow restricted areas of the secondary side components has not been as efficient as desired. Such flow restricted areas include the annular gap between the heat transfer tubes and the tubesheet, as well as gaps between the tubes and supporting devices for the tubes, or separator plates. In U.S. Pat. No. 4,257,819, a process is described for flushing out a narrow gap, such as the gap between a heat transfer tube and a tubesheet of a steam generator. As described therein, clean water, or alternatively, an organic solvent, are added to the secondary side, the water pressurized to about 3 atmospheres by an air compressor, and localized heating, by a heating device, is applied to the bottom of the gap between a heat transfer tube and the tubesheet. The pressure in the secondary isde is then reduced to cause flashing of water in the gap. Alternately, the repeating of pressurization and reduction in pressure can be used. Such a method is intended to flush the tubesheet crevice annulus.